1) Field of the Invention
The invention relates to a thermally insulating jacket under reversible vacuum, having a heat-transfer coefficient which changes depending on the different possible situations during the usage, requiring sometimes a fair thermal insulation and sometimes, on the contrary, a rapid heat dissipation.
A jacket of this kind had already been proposed for insulation e.g. of a heat accumulator; see U.S. No. Pat. 3,823,305. Such a jacket was consisting of an inner wall, an outer wall and a hollow space between the two walls; one of the walls of an insulating jacket (in this case the inner one) has obviously to be hotter than the other. Said hollow space was typically containing:
(I) a reversible hydrogen getter, namely a getter which can release or reversibly re-adsorb minor or major amounts of hydrogen, depending on the temperature employed for heating or respectively cooling the getter; PA1 (II) a first amount of hydrogen, chemically adsorbed by said getter in the solid state and depending on the getter temperature; PA1 (III) a second amount of free gaseous hydrogen, which takes up the whole available volume inside the jacket; also this second amount depends on the getter temperature. PA1 ZF=33% V=33% Fe=balance. PA1 a) the hydrogen release and/or re-adsorption (especially this last passage) are too slow for applications which be at an actually industrial level; PA1 b) said release and/or re-adsorption are even more slow if there is a considerable amount of carbon monoxide or of the other gases, different from hydrogen, usually present in a vacuum chamber (CO.sub.2, H.sub.2 O, O.sub.2, N.sub.2, CH.sub.4); PA1 c) the hydrogen release rate and the hydrogen re-adsorption rate, not very high per se since the beginning, decrease rather quickly with the time; it is namely possible to observe a degradation with the time of the reversible gettering activity with respect to hydrogen. PA1 a) said reversible hydrogen getter has a hydrogen equilibrium pressure Px.sub.1 lower than 10,000 Pa (preferably 1,000 Pa) at 500.degree. C., when the hydrogen concentration in the getter is 0.1% b.w., and is kept at a variable or constant temperature Ti essentially different from the temperature Tc of the hotter wall of the jacket; PA1 b) said hollow space contains a non-evaporable promoter getter having a hydrogen equilibrium pressure Px.sub.2 higher than 10,000 Pa (preferably higher than ten times Px.sub.1 and even better higher than one hundred times Px.sub.1) at 500.degree. C., when the hydrogen concentration in the getter is 0.1% b.w., which is essentially exposed to said temperature Tc. PA1 a) 78% Zr--20% V--2% Fe PA1 b) 45% Zr13 20% V--35% Fe PA1 c) 48% Zr--50% V--2% Fe PA1 d) 70% Zr--35% V--5% Fe PA1 e) 70% Zr--24% V--6% Fe PA1 f) 66% Zr--24% V--10% Fe PA1 g) 47% Zr--43% V--10% Fe PA1 h) 47% Zr--45% V--8% Fe PA1 i) 50% Zr--45% V--5% Fe. PA1 Zr=70% V=24,6% Fe=balance. PA1 the cylindrical shape; PA1 the hemicylindrical shape; PA1 the shape of two hemicylinders, wherein the first is under reversible vacuum and the second is under stable vacuum or under reversible vacuum.
The hotter the getter the greater the amount of hydrogen shifting from the adsorbed state to the free gaseous state; the colder the getter the lower the amount of free hydrogen, hence its pressure. The higher the hydrogen pressure the higher, within certain limits, the heat-transfer inside the jacket.
A particular field of application of these jackets having hydrogen depending heat-transfer properties are the electrical accumulators installed on battery-driven cars, even if in a semiexperimental or testing phase. Such batteries, as known, are hot working (300.degree.-425.degree. C.) and are generally consisting of a couple lithium/sulphides (425.degree. C.), sodium/sulphur (325.degree. C.) or sodium/nickel chloride (300.degree. C.). These batteries must quickly disperse the heat in case of overheating, which occurs, depending on the kind of battery, during the discharge phase, as in the case of the sodium/sulphur elements, or during the recharge phase, in the case of the lithium/sulphides batteries.
Apart from these situations, which repeatedly occur, in a cyclical way, during the normal run of a battery, it is also possible to observe emergency situations, as in the case e.g. of a rapid discharge or other, which could lead to a sudden overheating of the elements. In such situations it is equally important to have an effective means or an effective expedient for rapidly increasing the heat dissipation through the hollow space of the jacket.
The overheating can be avoided, as is known, by increasing very quickly the hydrogen pressure in the hollow space of the jacket because of the high thermal conductivity of H.sub.2. Viceversa, when the conditions engendering the overheating are failing, it is necessary to minimize the heat dispersion in order to avoid a lowering of the temperature of the elements below the optimum efficiency level (300.degree.-425.degree. C.).
All this can be realized by restoring the low-pressure conditions in the hollow space of the jacket, by letting the reversible getter reabsorb the hydrogen. The thus obtained vacuum, on the other side, tends as is known to worsen with the time and it is therefore indispensable not only to rapidly create a satisfactory vacuum degree but also to grant the maintenance of said vacuum degree as long as possible.
The double requirement hereinabove (rapid increase and respectively rapid decrease of the hydrogen pressure) can be fulfilled, as a first approximation, by placing a non-evaporable reversible hydrogen getter in an insulated housing outside the jacket and in fluid communication with the same jacket.
An example of such a positioning of the getter according to the known technique is illustrated in FIG. 1. When it is necessary to maximize the insulation, the getter is cooled to room temperature and the hydrogen pressure is consequently lowered, for instance to a level below 1-0.1 Pa in the case of a battery-driven car, whereby the heat dissipation is limited. When it is necessary, on the contrary, to promote the heat dissipation, it is necessary to use an outer or inner electric heating device, which raises the temperature of the getter material; a considerable amount of hydrogen is thus released, which makes the hydrogen pressure rise even up to 1000 Pa, in the case of a battery-driven car.
Again, in this case of the battery-driven cars the expression "reversible vacuum" defines the possibility of shifting the vacuum from a minimum value of the working pressure equal or lower than 5 Pa, preferably 1 Pa and even better 0.1 Pa, to a maximum operative value equal or higher than 50 Pa and up to 1000 Pa.
2) The Prior Art
A non evaporable reversible hydrogen getter tested in the past was consisting for instance, of one alloy (Zr--V--Fe) containing (% by atoms):
However, also this reversible alloy does not still provide quite satisfactory results as to the manufacture, for instance of battery-driven cars; at least the following drawbacks can be in fact registered:
Other getters were suggested by U.S. Pat. No. 4,455,998, that discloses the use in combination of a titanium-niobium alloy and of zirconium hydride; in this combination the titanium-niobium alloy acts as a reversible getter material for hydrogen, while zirconium hydride is an irreversible getter for oxygen, that releases hydrogen upon absorption of oxygen. However, the speed of hydrogen release and re-absorption of this combination of materials is too low, and far from being satisfactory from an industrial point of view.
A first object of the instant invention is to promote the hydrogen release rate and/or the hydrogen re-adsorption rate from and respectively by said reversible hydrogen getter.
A second object of the instant invention is to grant the creation and the maintenance of a fair vacuum degree in the hollow space of the insulating jacket hereinabove.
A third object of the instant invention is to extend in the time the action of said reversible getters, thus allowing said vacuum degree and said release and re-adsorption rate to long and steadily last at a high level.
A further object of the present invention is to keep said rates at a high level even in the presence of considerable amounts of carbon monoxide and/or other residual gases usually present in a vacuum.
Still another object of the present invention is to provide a battery for automotive traction purposes maintained at the optimum working temperature by means of the jackets of the invention.